The overall objective of this research proposal is to understand the mechanism by which epigenetic processes such as DNA methylation can contribute to neoplasia. We wish to investigate a direct causal role for DNMT1, the major mammalian maintenance DNA (cytosine-5) methyltransferase, in inducing C-T transition mutations similar to that shown for bacterial (cytosine-5) methyltransferases. This will involve the creation of conditional and tissue-specific alleles of DNMT 1 and the generation of transgenic mice and primary murine embryonic fibroblast (MEFs) cell lines using the established Cre/LoxP or Flp/Frt system and standard genetic methods. We will define the role of Dnmt1 in the incidence of point mutations in MEFs harboring the various conditional alleles by performing a quantitative analysis that measures spontaneous and induced mutation frequencies. We will then analyze wild-type and tumor prone mice that contain tissue-specific and conditional alleles of DNMT1 for the incidence of genomic methylation, incidence of point mutations, and tumor formation. These studies may provide a novel paradigm for understanding events that lead to abnormal cellular proliferation.